The present disclosure generally relates to methods for purifying a 2-aryl-3,3-bis(hydroxyaryl)phthalimidine.
Phenolphthalein derivatives have been used as aromatic dihydroxy compound monomers for preparing polycarbonate polymers as well as polyarylate resins characterized with excellent ductility and high glass transition temperatures. Polycarbonate homopolymers have been prepared by an interfacial polycondensation method using phosgene and 2-hydrocarbyl-3,3-bis(4-hydroxyphenyl)phthalimidine monomers, example, 2-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine (hereinafter sometimes referred to as “para, para-PPPBP”).
Lin and Pearce (Journal of Polymer Science: Polymer Chemistry Edition, (1981) Vol. 19, pp. 2659-2670) reported the synthesis of para, para-PPPBP for preparing polycarbonates and other polymers by refluxing phenolphthalein and aniline hydrochloride in aniline for 6 hours followed by recrystallization from ethanol. During this reaction, side products are created which, if not removed, can result in para, para-PPPBP having an unacceptable color and purity for use as a monomer or as a comonomer. The undesirable side products or impurities generally include both inorganic and organic species. With regard to the manufacture of polycarbonate, the impurities can result in a polymer having a high color, such as a yellowness index of greater than 10; or hinder polymerization and result in low weight average molecular weight polycarbonates, e.g., a number average molecular weight less than about 22,000 Daltons for melt polymerization, which can result in undesirable physical properties to the resulting polymer, such as increased brittleness. Furthermore, the impurities in the para, para-PPPBP monomer can include, for example, trace (parts per million) levels of phenolphthalein and phenolphthalein-derived organic compounds, and/or parts per billion levels of metals that can undesirably produce discoloration in the polycarbonates and other polymers derived therefrom, thereby affecting the transparency of the polymer product. Coloration is not desirable for many commercial applications. U.S. Pat. No. 5,344,910 discloses that copolymers of para, para-PPPBP were found to have poor melt stability resulting in foamy polymer melts and moldings, and discoloration of the resin during the melt processing.
It would therefore be desirable to develop a process for preparing purified 2-aryl-3,3-bis(4-hydroxyaryl)phthalimidines such as the para, para-PPPBP noted above that can be used to produce a polycarbonate having a yellowness index of less than 10. Further, such purified 2-aryl-3,3-bis(4-hydroxyaryl)phthalimidines can then be used for producing polycarbonates and other polymers having improved properties such as a higher number average molecular weight and a yellowness index less than 10.